Known in the art is an acoustic wave resonator having a piezoelectric substrate and an IDT (InterDigital Transducer) electrode provided on a major surface (the broadest surface of a plate-shaped member (front surface or back surface)) of the piezoelectric substrate (for example, Patent Literatures 1 to 3). The IDT electrode has a pair of comb-shaped electrodes. Each of the comb-shaped electrodes has pluralities of electrode fingers which extend parallel to each other. The pair of comb-shaped electrodes are provided so that their electrode fingers are alternately aligned in the direction of propagation of the acoustic wave. That is, the pair of comb-shaped electrodes are provided so that they intermesh with each other.
In the acoustic wave resonator as described above, pitches of the pluralities of electrode fingers (electrode finger pitches) are basically made constant. When voltage is applied to the pair of comb-shaped electrodes, an acoustic wave having the electrode finger pitch as a half wavelength (λ/2) is excited on the piezoelectric substrate. The frequency of this acoustic wave becomes a resonance frequency (fr). Further, an antiresonance frequency (fa) is defined by the resonance frequency and a capacity ratio (γ) of the IDT electrode (fa=fr×√(1+1/γ)). Here, the capacity ratio γ of the IDT electrode is represented by a ratio (C1/C0) of C0 representing mechanical vibration when replacing the IDT electrode with an equivalent circuit and an electrostatic capacity C1. The difference between the resonance frequency and the antiresonance frequency (Δf=fa−fr), for example, when configuring an acoustic wave filter by the acoustic wave resonator, exerts an influence upon the steepness of change of the attenuation amount at a boundary between the passing band and the bandwidth outside of that.
Although particularly not referred to the citations, when desiring to make Δf smaller, a capacity element is connected in parallel with respect to the IDT electrode. When such a capacity element is provided, the capacity ratio of the IDT electrode appears to become larger, therefore the antiresonance frequency becomes smaller relative to the resonance frequency. Consequently, Δf becomes smaller.
Note that, Patent Literatures 1 to 3 are not arts aimed at reduction of Δf. These citations propose provision of areas among which electrode finger pitches are different from each other or provision of a narrow pitch part in which the electrode finger pitch is smaller than in other parts in the IDT electrode.